@rjc430404 Searching the WordPress core codebase, I can't find _any_ reference to "ValidationService". Can you verify you are running the twentyseventeen theme, and also check that you don't have any "MU" plugins installed?

Can you tell in the inspector what file the error occurs in? You may need enable script debug to see where the error is (define( 'SCRIPT_DEBUG', true );) I still suspect this is not a WordPress core issue.

I know nothing of how core concatenates, substitutes or otherwise allows other plugins to insert values here, but I assume what @rjc430404 has encountered is either because the WP core code can be filtered to allow the insertion of something like "ValidationService" or because something naughty has happened to the install.

The unminified code is at line 1169 of /wp-includes/js/media-models.js:

// Observe the central `wp.Uploader.queue` collection to watch for
// new matches for the query.
//
// Only observe when a limited number of query args are set. There
// are no filters for other properties, so observing will result in
// false positives in those queries.
allowed = [ 's', 'order', 'orderby', 'posts_per_page', 'post_mime_type', 'post_parent' ];
if ( wp.Uploader && _( this.args ).chain().keys().difference( allowed ).isEmpty().value() ) {
this.observe( wp.Uploader.queue );
}

Again, not an expert at all, but a quick google shows the vast majority of "ValidationService" hits being related to angular.js. Is there a chance your theme or server are running angular?

My itchy spot says this seems like a hosting or machine-specific install of WP which is doing some undoubtedly lovely angular stuff.

@rogerlos ah, thanks for pointing out the minified file use, I was only looking thru the source code.

I am going to dig into this a bit further.

@rjc430404 can you describe the exact steps to reproduce the error? Anything unusual about your media library that might be relevant? Also, can you try adding define('SCRIPT_DEBUG', true ); to your wp-config.php file so you can see the error in the unminimized js files? Thanks